1. Field
The present invention relates to a photosensitive coloring composition, a color filter substrate produced by using this photosensitive coloring composition, and a liquid crystal display device provided with this color filter substrate. As for specific examples of the liquid crystal display device, they include a semitransmissive type liquid crystal display device which is capable of displaying images not only by means of transmitted light from a light source built into the display device but also by means of reflected light to be obtained from the reflection of external light such as sunlight or indoor light.
2. Description of the Related Art
A color liquid crystal display device is typically constructed as shown in the cross-sectional view of FIG. 1, wherein a liquid crystal 3 is encapsulated in a space between a color filter substrate 1 and an array substrate 2. The color filter substrate 1 comprises a transparent substrate 11 as a structural support, and a polarizing film 12 which is arranged on the observer's side surface of transparent substrate 11. Further, the opposite side (back face side) of transparent substrate 11 is partitioned into a large number of pixel regions wherein spaces between these pixel regions are filled with a light-shielding film 13 and these pixel regions are respectively provided with a transparent colored layer 14. This transparent colored layer 14 is designed to apply a color to the transmitted light passing through each of the pixels, so that transparent colored layers 14 of three different colors, i.e. red (R), green (G) and blue (B) corresponding to three primary colors of light, are generally arrayed for each of these pixels. Incidentally, the aforementioned light-shielding film 13 is designed to prevent the mixing of colors of transmitted light that has been colored by any one of these colors.
The color filter substrate 1 is further provided with an over-coat layer 15 for burying the step portions that have been formed by the transparent colored layers 14. Thereafter, a transparent electrode 16 and an orientation film (not shown) are successively formed on the over-coat layer 15, thereby accomplishing the color filter substrate 1.
On the other hand, the array substrate 2 which is disposed to face the color filter substrate 1 comprises a transparent substrate 21 as a structural support. Electrodes 23 and an orientation film (not shown) are arranged on the liquid crystal side surface of the transparent substrate 21, and a polarizing film 22 is arranged on the opposite side surface of the transparent substrate 21.
The pixels are designed to be respectively impressed with a voltage applied between the transparent electrode 16 and the electrodes 23, thereby controlling the transmission or non-transmission of light with the transmitted light being taken up as a display light for displaying images.
There are known various methods with respect to the method of forming the transparent colored layers 14 of the color filter substrate 1. Among them, the method which is now most commonly employed is a method using a negative type photosensitive coloring composition, wherein the coating film of this photosensitive coloring composition is subjected to the exposure of a proximity exposure system using a proximity aligner. More specifically, a photosensitive coloring composition is coated on a surface of a transparent substrate 11 having the aforementioned light-shielding film 13 formed thereon to form a coated film, to which ultraviolet rays are irradiated through a photomask which is provided with a light-shielding layer patterned corresponding to the transparent colored layers 14, wherein the photomask is disposed at a location spaced away from the coating film by a distance ranging from several ten to several hundred micrometers. As a result, the coating film is selectively exposed and cured. Then, the resultant coating film is subjected to a developing process using a developing solution to remove unexposed portions (un-cured portions) of the coated film, thereby accomplishing the transparent colored layers 14. Incidentally, the aforementioned procedures are repeated with respect to each coloring photosensitive coloring composition, thereby making it possible to form transparent colored layers of three colors 14R, 14G and 14B.
Meanwhile, the liquid crystal display device can be roughly classified into a device which utilizes, as a display light, the light of a light source such as a back-light, which is disposed on the rear side of the array substrate 2, and a device which utilizes, as a display light, the light that can be derived from the reflection of external light, such as outdoor light. The former is called a transmissive type liquid crystal display device, which is accompanied with a problem that, although it is capable of displaying a bright image, it requires a large magnitude of power since the light source thereof is required to be built in the interior of the device. On the other hand, the latter is called a reflective type liquid crystal display device, which is accompanied with a problem that, although it requires a relatively small magnitude of power, it is difficult to obtain a bright display image in a room where there is insufficient external light.
For these reasons, there has been proposed a liquid crystal display device which makes good use of not only the advantages of the transmissive type liquid crystal display device but also the advantages of the reflective type liquid crystal display device, whereby a bright images can be displayed by making use of the light of a built-in light source in a room while minimizing the power consumption through the utilization of external outdoor light. This liquid crystal display device is called a semitransmissive type liquid crystal display device. A semitransmissive type liquid crystal display device is already actually employed as a display device in mobile instruments, such as a cellular phone or digital still camera.
However, there is a problem with this semitransmissive type liquid crystal display device, as follows. Namely, when the light of a light source is to be utilized as a display light, this display light is caused to pass through the transparent colored layers 14 only once. Whereas, when the reflected light to be derived through the reflection of external light is to be utilized as a display light, this display light is caused to pass through the transparent colored layers 14 back and forth, i.e. twice. For this reason, when the reflected light to be derived through the reflection of external light is to be utilized as a display light, the loss of light is caused to increase due to the transparent colored layers 14, thus resulting in increased darkness of the display image.
There have been proposed various methods to overcome the aforementioned problem to thereby secure a brighter display image. For example, there is known a method wherein the pixel region is partitioned into two sections, i.e. a reflection section “b” and a transmission section “a”, the reflection section “b” being utilized in a case where the reflected light is used as a display light for displaying an image, and the transmission section “a” being utilized in a case where the light from a light source is used as a display light for displaying an image. Namely, a reflective film is selectively provided at the reflection section and the external light entering this reflection section is utilized as a display light. On the other hand, the light from light source that can be derived from the rear side of the transmission section “a” is permitted to pass therethrough and be utilized as a display light.
The pixel regions of color filter substrate 1 of this semitransmissive type liquid crystal display device are constructed as shown in FIG. 2. Namely, the transparent colored layer 14 which is located at the reflection section “b” is provided with a through-hole 14x, so that the transparent colored layer 14 does not exist in this through-hole 14x. Because of this, the display light passing through this through-hole 14x is free from any loss of light based on the existence of transparent colored layer 14, thereby making it possible to create a bright display image. The transparent colored layer 14 provided with this through-hole 14x can be also formed by a method wherein a negative type photosensitive coloring composition layer is subjected to exposure through a photomask provided with a light-shielding film having a corresponding pattern.
Incidentally, the display device for the aforementioned mobile instruments is also demanded to have a display image of higher definition, and hence the aforementioned through-hole 14x is also demanded to be of higher fineness. For example, if it is desired to enhance the resolution of a 2.4-inch type cellular phone with a conventional QVGA (320×240 pixels) resolution up to as high as QVGA (640×480 pixels), it is required to reduce the width of individual pixels of the RGB material layer from about 75 μm down to about 25 μm. Therefore, the through-hole to be formed in a pixel having a width of 25 μm is also required to be smaller in size. For example, the size of the through-hole is required to be decreased down to 10 μm or less in diameter.
However, when the exposure of a coated layer of a photosensitive coloring composition of a negative type is to be performed by making use of a photomask which is provided with a finely patterned light-shielding layer in order to form a color layer having such fine through-holes, diffraction of light is generated from the edge portions of light-shielding layer of the photomask, and the coated layer of the photosensitive coloring composition of the negative type is sensitized by this diffracted light, thereby raising a problem that the diameter of these through-holes cannot be accurately controlled.
Since the intensity of the diffracted light is generally smaller than the intensity of transmitted light (zeroth-order diffracted light), it is conceivable to accurately control the diameter of through-holes by taking advantage of this difference in intensity between the diffracted light and the transmitted light. Namely, based on an exposure sensitivity curve derived from the plotting of exposure sensitivity with the abscissa representing the common logarithm of the exposure applied to a coated layer of the photosensitive coloring composition of the negative type and the ordinate representing the residual rate of the film after development that, as the tangent of rising angle θ (tan θ) becomes higher, the contrast between the exposed portion and un-exposed portion after development becomes higher, thereby making it possible to enhance the resolution of through-holes. The details related to this phenomenon are described in “General Review on Latest Polymer Materials and Technology”; 37 (1988); Ishikawa et al., Tech. Publ. Co., Ltd.; and in “Organic Electronics Materials”; Science Forum; 15 (1986); Taniguchi et al.
When the exposure of the coated layer of a photosensitive coloring composition of the negative type is performed with the minimum possible quantity required of exposure by taking advantage of this principle, the coated layer would be sensitized by the transmitted light (zeroth-order diffracted light) while making it possible to prevent the coated layer from being sensitized by diffracted light which is lower in intensity than the transmitted light. Subsequently, when the sensitized coated layer is developed, it would be possible to form a transparent color layer wherein the residual rate of the film differs between the exposed portion and un-exposed portion. Accordingly, it is conceivable, in this manner, to accurately form through-holes each having a refined diameter.
However, it has been found out as a result of studies made by the present inventor that it was difficult to accurately create through-holes each having a diameter of 10 μm or less if only the tan θ was controlled as described above. For example, when the photosensitive coloring composition of the negative type was highly sensitive and hence the minimum possible quantity required of exposure was small, the coated layer of the photosensitive coloring composition of negative type was rendered sensitive also to diffracted light and, because of this, it was found difficult to accurately form through-holes each having a fine diameter.